Add requireness to HasOne form

[goosys]

The nested form for HasOne didn't have the requiredness mark, so I added it.

Before:

After:

[nickcharlton]

Can you think of a reasonable way to test this, so we don't break it in future?

[goosys]

I will think of a reasonable way to test this and add it!

[goosys]

I've added tests.

By the way, ProductoMetaTag is the only place in the demo app that uses Field::HasOne, but it has too few columns, both of which are String type and required. This makes it less suitable for comprehensive testing.

Would you consider adding more columns?

[goosys]

Regarding this method name, is "requireness" correct? Should it be "requiredness" instead?

[nickcharlton]

By the way, ProductoMetaTag is the only place in the demo app that uses Field::HasOne, but it has too few columns, both of which are String type and required. This makes it less suitable for comprehensive testing.

We should add more if it's needed for testing. The demo app has grown organically and is neither consistent nor coherent in how it works. I've long wanted to switch the demo app to follow a particular pattern. Maybe a shop or something.

Regarding this method name, is "requireness" correct? Should it be "requiredness" instead?

I think "requiredness" is likely more correct, according to the OED it is a word, although not a common one.

[goosys]

Rebased!

[nickcharlton]

Suggested change

	<div class="field-unit field-unit--<%= attribute.html_class %> field-unit--<%= requireness(attribute) %>">
	<div class="field-unit field-unit--<%= attribute.html_class %> field-unit--<%= requiredness(attribute) %>">

I think we should probably make the change to "requiredness", as I do think that's more correct than "requireness".

[goosys]

Should this be changed in this PR?
Or would it be okay if I open a new PR?

[nickcharlton]

Oh! Right, it's not actually introduced here. I hadn't realised. Let's leave it for now, we've already got the existing name and it's fine.